FossaMail is a free, open source, mail, news and chat client for Microsoft Windows and Linux (available in both 32-bit and 64-bit versions) based on Mozilla Thunderbird. It is a re-build of Mozilla Thunderbird ESR (Extended Support Releases) that has been optimized and reconfigured by the same developer behind Pale Moon browser. Is built from source with the main goal of providing a fast and reliable Mozilla Thunderbird-based client that should work with all existing add-ons and plugins that were developed for Mozilla Thunderbird. It uses its own profile folder (default location to "%APPDATA%\FossaMail") and therefore has no interaction with any current installation of Mozilla Thunderbird. Please note that Windows XP is not supported and you need at least Windows Vista (or a newer edition of Windows), a CPU (processor) with SSE2 support (most modern CPUs support this, check this section - "CPUs supporting SSE2") and obviously, enough space on your computer hard drive to store your e-mails along with an Internet connection.
Note: From this page you can download the "Lightning add-on" for both x86 (32-bit), x64 (64-bit) versions. Currently, the FossaMail backup tool can be downloaded from here.
The first stable version of FossaMail is version 24.5.0 (the one that I tested) and it was difficult for me to separate it from the same version of Mozilla Thunderbird.
The best way to compare them is to install both applications on your computer and test each. That's exactly what I did, I installed FossaMail and then, Mozilla Thunderbird (24.5.0 - this was the latest stable version at the time that I wrote this).
Software interface: Alright, they both look "almost" the same. I said almost because the first thing that I noticed was the existence of both "Mail Toolbar" and "Menu bar" in FossaMail while at Thunderbird, the "Menu bar" is disabled by default. Again, it's obvious that both of them share the same UI (User Inteface) but FossaMail has better icons (you can probably customize Thunderbird to look the same).
Speed: I tested both clients on Windows 7 (the Linux versions were not tested) and I couldn't notice any differences when it comes to speed. Those who run an older computer, might note a small difference between them. After all, the developer seem to follow the same path as he used for Pale Moon which is optimized to work better than Firefox so the same principle applies here.
Conclusion: Right-now, I am looking at both clients, I have two windows opened, connected to the same email account (I made the test on Yahoo Mail and I have no idea if other email providers allow it - they should, considering it's from the same IP address) and I tried to notice any difference between them. At Thunderbird, there are several minor things such as a message that ask for sending reports to Mozilla, a help section right-under the mail email account and probably other stuff that I didn't notice. FossaMail doesn't have those (enabled by default) and I bet that most of the changes are, as the author describes them "under-the-hood" not just at the user interface level. Anyhow, you shouldn't look at this client as an inferior product. In my opinion, if you look for an alternative to Mozilla Thunderbird or better said, something built on it but with a slightly different flavour, try FossaMail.. If there's a positive impression for this Open Source product, please consider a donation to the developer for his work.